Nowadays semiconductor power management chips are broadly applied to portable electronic products like mobile phones, PDAs, notebook computers, etc. In a development trend of system on chips the framework of single-inductor multi-output DC/DC converter is adopted in order to reduce the chip size. However, the multi-output framework has problems of poor stability and cross regulation effect.
Traditionally, multi-outputs are mutually separated such that the output terminals are not interfered by the change of other loads therefore solving the problem of cross regulation effect. A Pseudo-CCM current technology may be used and the whole system is like in a Discontinue Current Mode (DCM) state, resulting in easy stabilization of the system. Moreover, as the system is provided with zero current equivalent to the DCM, and each switching period is provided with a buffer state, a momentary change of load does not affect next switching period therefore reducing the cross regulation effect. However, a Freewheel stage must be added in during the whole PWM period; as the switch is not conducted ideally, a large amount of power can be consumed by the equivalent resistance of switching in this stage; therefore, the Conduction Loss of the whole system can be increased, thereby reducing the efficiency of conversion. Besides, the energy stored in the inductor during the Freewheel stage can not be transferred to the output terminal and the average inductor current is more than the sum of the output loads. A larger average inductor current can cause a larger output voltage ripple due to the discontinuous characteristic of the output inductor current of the single-inductor multi-output module framework; therefore, a high-efficiency post-voltage stabilization circuit is needed to further process the output voltage.
Another method refers to adopt a priority energy distribution flow. However, this method is only applicable to a certain specific load sate; moreover, the method uses a comparator to control the output voltage and does not provide a satisfactory overall voltage stabilization effect compared with the effect of a close loop which adopts an error amplifier to control.
Besides, a framework combining the inductor and a Charge Pump can be used. However, the framework shall additionally use an external capacitance and a diode, and can have a larger output voltage ripple. Moreover, as the negative-voltage output is achieved by the Charge Pump, the negative-voltage output can have a poor voltage stabilizing situation and is quite undesirable in actual application.
In light of the above technical difficulties, this invention provides a power conversion system and power control method for reducing cross regulation effect to resolve the above difficulty by simultaneously combining the electronic and circuit technologies and energy control concept.